Which of the following is the formula for the geometric sequence?
2, 6, 18, 54, ...
A. an = 3(2)^n-1
B. an = 2(4)^n-1
C. an = 2(3)^n-1

Geometric progression follows the rule
a, ar, ar^2, ar^3 and so on
comparing first two terms, you can find out that r = 3
you can see that since the first term (a) = 2, the possible answers can be B or C
Since you have found r, you know that the answer is C